Plasma heating method

ABSTRACT

An electric furnace adapted to be heated with a gaseous plasma, and including within the furnace a quantity of low work function material which promotes the establishment and sustenance of the gaseous plasma, said quantity being on the order of about one milligram per square centimeter of furnace area.

0 United States Patent 1 [1 1 3,71 1,615 Petersen et al. 1 51 Jan. 16, 1973 PLASMA HEATING METHOD 3,479,022 11/1969 Coupette ..13/31 X [75] Inventors: Donald H. Petersen, Dallas; warren 3,524,006 8/1970 Ebelmg et al ..l3/l X C. Schwemer, Arlington, both of 1 Tex. Primary Examiner-Bernard A. Gilheuny [73] Assignee: Advanced Technology Center, Inc., ASH-am Envull'jr' Grand prairie Attorney-C. W. McHugh and H. C. Goldwire [22] Filed: Sept. 30, 1969 Appl. No.: 871,206 ABSTRACT Related Us. Applicafioh Data An electric furnace adapted to be heated with a gaseous plasma, and including within the furnace a quantil l 3 0f J 1963, ty of low work function material which promotes the 9 establishment and sustenance of the gaseous plasma, [52] U S Cl 13/1 13/31 said quantity being on the order of about one milli- [51 1 h. .0 E05,) 7/00 gram per square centimeter of furnace area [58] FieldofSearch ..l3/l,3l,9;2l9/l2lP [56] References Cited 7 Claims, 6 Drawing Figures UNITED STATES PATENTS 3,106,594 10/1963 Beasley et a] .f. ..13

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PATENTEUJAHBIQYS 3.711.615 SHEEI 2 ur 2 ARGON-NITROGEN 3 (M BY VOLUME) I o 4 E ARGON O. 3 3

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| l l l I 20 30 4o 50 so 70 ELECTRODE POTENTIAL, VOLTS DONALD H. PETERSEN WARREN C. SCHWEMER, INVENTORS FIG 6 BY PLASMA HEATING METHOD This is a division of application Ser. No. 786,140 filed Dec. 23, 1968, and now U. S. Pat. No. 3,520,977.

This invention relates to electric heating means and more particularly to means for heating an enclosure with a gaseous plasma.

The use of a plasma generator to provide a gaseous stream at very high temperatures is well known, and even relatively sophisticated devices such as that described in U. S. Pat. No. 2,972,696 to A. R. Kantrowitz, et al. have been designed. Devices such as the Kantrowitz generator, however, suffer from the difficulty encountered in re-establishing an arc between two spaced electrodes if the arc is deliberately interrupted (as a means of temperature control) or is inadvertently interrupted due to a momentary power failure, etc. That is, plasma generators of the type described by Kantrowitz, et al are generally serviceable only when a continuous electrical discharge between electrodes is tolerable or desirable.

While the U. S. Pat. No. Re 25,958 to J. M. Beasley et al. discloses a method of heating an enclosure with a gaseous plasma in which an arc can be re-struck with ease if it is ever interrupted, the Beasley et al. furnace also suffered from a liability, namely, a temperature limitation. That is, the Beasley et al furnace is re-startable without bringing the hot electrodes back into very close proximity only if the temperature in the furnace is never allowed to drop below 2800F. Thus, as stated by Beasley et al. at column 8, lines 38-47, the argon-enriched atmosphere in the furnace interior must be kept at or above 2800F in order to realize the advantageous re-starting characteristics of the furnace.

Hence, it has been shown that plasma heating devices in general are old. Furthermore, it has been shown that such devices can be adapted to heat the interior of the type of furnace that might be used for melting materials having relatively high melting temperatures. It has also been shown that an arc in a plasma furnace can be restruck without bringing the electrodes together as long as the atmosphere in the furnace is enriched with a noble gas and the furnace interior is maintained at or above 2800F. But, heretofore, a method of achieving the advantages of a restartable plasma furnace has not been disclosed which obviated the need to maintain the furnace at a relatively high temperature of 2800F.

Accordingly, it is a major object of this invention to provide a plasma furnace which is re-startable at temperatures as low as I650F.

It is a further object to provide a plasma heating device which is stable at temperatures as low as 1300F.

Another object is to provide a low-temperature plasma furnace which is operable with a wide variety of gases.

A further object is to provide a method of maintaining a furnace at a desired temperature in a manner such that the furnace is characterized as being self-regulatmg.

Yet another object is to provide a means for promoting the establishment of a gaseous plasma in an areheated enclosure.

These and other objects and advantages will be apparent from the specification and claims and from the accompanying drawing illustrative of the invention.

In the drawing,

FIG. 1 is a sectional view in elevation of a furnace in which the present invention can be practiced;

FIG. 2 is a plot of plasma temperature versus plasma resistance in a furnace similar to that shown in FIG. 1, and wherein the gas in the furnace cavity is helium;

FIG. 3 is a sectional view in elevation of an alternate configuration of a furnace cavity, in which a horizontal ledge is provided on which material may be placed to separate it from the melt;

FIG. 4 is a sectional view in elevation of an alternate furnace configuration in which the material to be melted can be isolated from the gaseous plasma;

FIG. 5 is a plot of operating voltage versus electrode gap for several gases; and

FIG. 6 is a sectional view in elevation of a furnace having holes in the walls which are filled with solid rods of low work function material.

With initial reference to FIG. 1, the furnace generally indicated by the numeral 10 has a housing 11 with a cavity 12 therein which is to be heated. Such a furnace 10 is described in great detail in U. S. Pat. No. Re. 25,958 to Beasley et al., and therefore only those parts which are truly essential for an understanding of the present invention will be repeated here. It should be noted, however, that while this invention is advantageously used with a furnace which will hereinafter be referred to as a Beasley-type furnace, it is also useable with any other furnace which is capable of supporting a gaseous plasma. That is, any furnace which-has two or more electrodes that are connectable to a source of AC or DC power, and also has a connection for admitting a gas which is capable of forming a gaseous plasma, will serve adequately as the nucleus for practicing the instant invention.

Extending into the cavity 12 are two electrodes 13, 14 which are separated by a gap 15. The pair of elec trodes 13, I4 quite obviously are surrounded by the gas in the cavity, so that the gas will be heated when an arc is struck between the electrodes. In those instances where it is desirable to establish an are between a first electrode and the charge or melt (which is in electrical contact with a second electrode), it should be apparent to those skilled in the art that functionally the charge/melt constitutes a physical (and electrical) extension of the second electrode. Hence, it is appropriate to refer to two electrodes extending into the cavity even if only one of the electrodes actually has the typical elongate shape illustrated in the drawing. The initial heating of the gas to a plasma-forming temperature is conveniently accomplished with the very same electrodes that operate to sustain the gaseous plasma, but initial heating could be achieved in some alternate manner without departing from the concept of the invention.

A source of AC or DC electrical power 16 is connected through suitable leads and insulated connectors to the two electrodes I3, 14. As will be explained more fully hereinafter, the power source 16 is preferably a constant-current power source such as an SCR-type power supply. Such a power supply is available from the Hewlett-Packard Company, 1501 Page Mill Road, Palo Alto, Calif, in their SCR-IO series of DC regulated power supplies which are characterized as providing constant voltage/constant current operation with automatic crossover. A switch 17 is provided to selectively permit the discharge of electrical energy across the gap 15. The showing of the electrodes 13, 14 by broken lines in FIG. 1 is for the purpose of indicating that the electrodes are movable from positions where they may be very close, or even in contact, to other positions where they are widely spaced. As is taught in the aforementioned Beasley et al. patent, if an arc is struck between the two electrodes 13, 14 while they are very close, and a suitable atmosphere is maintained between the electrodes at a proper temperature, the gas between the electrodes will become ionized and will act like commonly known conductors of electricity. When this condition is reached, the electrodes l3, 14 may be separated until essentially all of the gas in the furnace is integrated into the heating circuit as an ionized, gaseous resistance element or plasma.

While operation of the Beasley furnace in accordance with the Beasley et a]. patent is restricted to operation with a noble gas (such as argon), or at least an atmosphere that is enriched with a noble gas, no such restriction is necessary with the present invention. That is, it has been found that when certain materials are present in the furnace (even in extremely small amounts), that a stable plasma condition can be established in a variety of gases that heretofore had been avoided for one reason or another. Accordingly, a gas source 18 which is initially if not continuously in communication with the cavity 12 through valve 19 is provided so as to establish a desired atmosphere in the cavity. Gases that are capable of forming a gaseous plasma in the present invention include argon, helium carbon monoxide, nitrogen, hydrogen, air, neon, and mixtures of all of these gases.

Of all of the structure thusfar described, there is relatively little that is novel. What is novel about the furnace and its manner of operation, however, cannot be readily seen in a drawing. Thus, the essence of the invention consists of the combination of the furnace described thusfar (or with any other furnace) with a means for promoting the establishment of a-gaseous plasma and for stabilizing or sustaining such a plasma once it is formed. Broadly speaking, this means consists of a quantity of low work function material which is placed within the cavity where, quite naturally, it is heated when the furnace is heated. Such material can be in a particulate form, and can be placed in the bottom of the furnace; or, it can be applied to the wall of the cavity 12 as a coating. Furthermore, it is not necessary that the low work function material initially cover all of the wall or walls of the cavity 12, although this may be the eventual result if some of the material vaporizes when it is heated and then condenses on what may be relatively cool walls of the cavity. Thus, it has been determined through experimentation that a coverage of the cavity wall of as little as 10 percent of the surface area of the wall is sufficient to promote stabilization of the plasma, with the coated spots being substantially uniformly distributed throughout the cavity. The distribution contemplated is somewhat like the manner in which squares of one color are distributed on a checkerboard.

Examples of what are referred to herein as low work function materials are the oxides of barium, calcium, strontium, radium, uranium, thorium, and materials thermally decomposable to form these oxides. However, any material which has a work function of 3 or less is believed to be serviceable and should be understood to fall within the scope of the appended claims. By work function, it is meant the voltage that is required to remove an electron from the surface of a material. The respective work functions of the preferred materials as reported in various scientific publications are as follows:

Material Work Function Barium oxide l.52.0 Calcium oxide 2.2

Strontium oxide 2.0

Uranium oxide 3.0

Thorium oxide 2.9

Although a value for radium oxide has apparently not been reported, it is predicted that it will be found to have a work function between 1 and 2, and should perform nicely in this invention, if certain radiation characteristics can be tolerated.

To explain why the preferred materials are properly denominated as low work function materials (with low being defined as no greater then 3), it should be recalled that, almost invariably, the materials from which furnace housings have been fabricated in the past have been magnesium oxide, silicon oxide,zirconium oxide and aluminum oxide, and mixtures thereof. These materials have work functions of 3.2 for magnesium oxide, 4.6 for silicon oxide, 3.4 for zirconium oxide and 3.8 for aluminum oxide. Hence, all plasma furnaces heretofore have had furnace walls made of material whose work functions are considerably above 3.

If the low work function material is to be applied as a coating to the walls of the cavity 12, it might seem that the thickness of the coating would be important to a person planning to practice the invention. It should be reassuring, then, to know thatthe thickness of the coating does not appear to be very critical, because re-starts have been obtained as readily at the beginning of a test run (when the coating is fresh and relatively thick) as they have at the conclusion of a test run (when a great deal of the coating has perhaps vaporized and leaked out of the furnace or has been absorbed by the porous walls of the housing). Even the initial thickness of the coating is not so critical as to be much of a limiting factor, because it is believed to be practically impossible to apply the low work function material as a coating which is too thin to promote stability of the gaseous plasma. For example, a coating which is so thin as to be nearly transparent has proven satisfactory, with such a coating being on the order of about 1 milligram per square centimeter of wall area. For those who find it difficult to visualize exactly how thin this is, such a thickness is roughly equivalent to the thickness of that small amount of chalk which inevitably remains on a conventional blackboard after the board has been thoroughly cleaned with a dry eraser.

As suggested earlier, the low work function material need not necessarily be affixed to the cavity wall; it can also be loosely placed in the bottom of the cavity, as long as the quantity of loose material is approximately equal to that quantity which would suitably cover the walls if placed thereon. Since placing the plasma-stabilizing material in the very bottom of the furnace cavity 12 could cause it to be at least temporarily shielded by the material to be melted as heating begins, it is sometimes advantageous to provide a trap or horizontal surface within the cavity and above the level of the materials to be heated. The plasma-stabilizing material is then placed on this horizontal surface instead of on the bottom of the cavity. Such a furnace configuration is shown in FIG. 3.

It may sometimes be desirable not only to separate the material to be melted from the low work function material, but also to isolate it from the gaseous plasma. To this end, a furnace having a housing 21 with two cavities 22, 23 therein is shown in FIG. 4. The first and outer cavity 22 is adapted to contain the gaseous plasma, while the surrounded, inner cavity 23 is adapted to contain the material to be heated. The heat which is generated in cavity 22 when electrical energy is discharged across the space separating the two electrodes is transferred through the common wall 24 between the cavities 22, 23 so as to raise the temperature within the cavity 23 to a wanted level. if desired, the walls of the housing 21 can be made of the low work function material, so that any labor which might be associated with the preparation of desired coating can be avoided.

Operation of an apparatus in accordance with the in vention begins with the step of providing a suitable gaseous atmosphere in the cavity which is to be heated. For example, if nitriding is to be accomplished with the furnace, the atmosphere should be rich in nitrogen (e.g., ammonia). Carburizing can be accomplished by exposing metal parts in the furnace to a carbon-containing gaseous plasma (as, for example, methane, acetylene, etc.). Sintering refractory oxides is conveniently accomplished by using an atmosphere of air or oxygen. If the furnace is to be used as a reactor for chemical synthesis, the atmosphere is naturally selected so as to be compatible with the synthesis to be accomplished. In this respect, the instant invention is markedly different from the Beasley furnace, since a wide variety of gases are usable, and notjust the noble gases recited by Beasley et al.

When the desired atmosphere within the cavity 12 has been achieved, it is heated to a temperature at which a plasma condition can exist. Assuming that the electrodes 13, 14 are to be used to strike an arc which will initially heat the gas in the cavity, the electrodes are brought sufficiently close to each other to permit an arc to span any gap that may exist between them. Once the arc is struck, the electrodes are then separated so that more and more of the gas between them is brought to very high temperatures. Eventually, the gas between the electrodes l3, 14 becomes ionized and a plasma condition exists which can encompass all of the gas in the cavity, not merely that gas which lies directly between the electrodes. The ionized gas acts like a heating element, as is described in the article entitled, Plasma Resistance Furnace," in the July, 1967, issue of Research/ Development.

The temperature at which the plasma will become stabilized is primarily a function of the current input, and an equilibrium temperature is most conveniently established by controlling the current rather than some other parameter. For a given gas, the stabilization temperature is also affected by any low work function material which is present, and may also be affected by the particular furnace construction, i.e., the furnace size, shape, and the insulation (which will affect the temperature of the cavity wall). If the walls of the cavity are cool, the furnace will not usually operate as efficiently as it will when the walls are hot. An explanation of the effect of 'cold walls in a plasma furnace is that they induce thermalization and de-ionization of the plasma, which naturally affects plasma stabilization adversely. Also, the plasma-stabilizing activity of the low work function material has been found to increase with increasing temperatures.

It has also been found that the plasma resistance is dependent on the plasma temperature in such a way that the furnace can be essentially self-regulating if the source of electrical power is a constant-current power source. To illustrate this, let it be assumed that a given temperature of, say, 3,000F has been selected as the desired temperature at which the furnace is to be stabilized. A power supply is then selected based on empirical data or general experience with similar furnaces to achieve this temperature. For example, a person skilled in the art will recognize that a power supply having a rating of about 25 kw would likely be adequate for a small furnace with a spherical cavity having a diameter of, say, 4 inches. Typical operating values of current and voltage for such a power supply would likely be 450 amps and 45 volts. If the furnace is larger than 4 inches in diameter, all the other items that affect the furnace would be scaled up roughly in direct proportion to the furnace size. it is reassuring that electrode spacing has not been found to be a limiting factor, and it is only required that the power supply be rated at about 50 volts per foot of maximum electrode gap in order to accomodate any of the gases described herein. If the gas to be used and the maximum electrode gap are known, the voltage requirement can be ascertained with greater certainty by reference to a curve like that shown in FIG. 5, which is a plot of operating voltages versus electrode gap for several gases. The slope of these curves indicates that very large gaps can be sustained with practical operating voltages. For example, with argon, a 12-foot gap would require only about 440 volts.

Once the needed current level has been identified and the power supply has been appropriately set, the plasma, once established, will tend to sustain itself without a change in current level. Thus, as shown in FIG. 4, the resistance of the plasma (helium in this case) increases with decreasing temperature. Assuming that the furnace is at a temperature represented by point A on the curve, and assuming the furnace cools off somewhat, the plasma resistance would go up, and the product of PR (which is the power consumed by the furnace) would go up, too. This increase in power consumption quite naturally will tend to make the temperature within the furnace move back towards point A. If the plasma temperature moves above the point A, the plasma resistance decreases, and the power consumption (l'R) goes down, such that the furnace will usually tend to cool off (because of inherent heat losses). The overall result is to stabilize the plasma at a temperature which is directly dependent on the current level set by the constant current power supply.

The curve drawn in FIG. 2 is the curve for helium in a furnace of the Beasley type, and without the benefit of any low work function material being present to artificially increase the supply of electrons throughout the cavity. By merely adding a small quantity of low work function material (such as barium oxide) to the cavity and allowing it to become heated, the entire curve is capable of being transposed downward by 200-500F and frequently by as much as lOOF. Correspondingly dramatic reductions in critical temperatures have been observed with other gases, both pure and mixed. Some of the other gases may not be as favorable for routine operation of the furnace, however, because of the relatively high cost, toxicity, etc. The quantity of low work function material referred to above is that quantity which would be required to cover the walls of the cavity with about 1 milligram of BaO per square centimeter of cavity wall. This quantity is so small that is has been truly difficult to measure it directly. The required quantity has been calculated after experimental results have shown that a quantity of about one gram of BaO placed in a cavity having nearly 700 square centimeters of surface area is sufficient to affect the plasma as described herein. ln one experiment, the barium oxide in particulate form was initially placed in the bottom of the cavity; but at the conclusion of the test, most of the barium oxide was found to have vaporized and subsequently to have been deposited on the walls of the cavity. Because of the difficulty of observing the interior of a furnace operating at these high temperatures, exactly when the barium oxide is vaporized and subsequently is captured at the cavity walls has not been determined. Thus, whether the barium oxide performs its highly beneficial function before it is vaporized, after some of it has vaporized but is still suspended with the plasma, or after it has vaporized and has subsequently condensed on relatively cool cavity walls, has not been determined. But, in spite of the fact that when has not been conclusively identified, the fact is known that the material does perform a valuable function.

It is worthy of note, however, that the phenomenon reported herein cannot be summarily dismissed as obvious in view of some theory related to electron tube operation. That is, the specific low work function materials referred to herein admittedly have been used in electron tubes for years, for the purpose of prom0ting electron emission. Thus, it is known that filaments coated with alkaline earth oxides are copious sources of electrons. But, a knowledge of filament coverage requirements and the deactivation of coatings when they react with impurities in an electron tube, would likely lead one to expect just the opposite of the phenomenon that has been observed in the furnaces as will now be explained.

Oxide-coated electron emitters are usually prepared by coating the surface ofa filament (normally nickel or tungsten) with a few mils of an alkaline earth hydroxide, nitrate or carbonate. Next, this coating is activated by a process which converts the metallic salt to an oxide which contains some free metal atoms. For exam ple, if the electrode is nickel and barium carbonate is the initial coating, the following reactions occur:

Baco, BaO co 8210 Ni NiO Ba If the electrode is tungsten and barium carbonate is the initial coating, the following reactions occur:

6 BaO w Ba WO 3 Ba Since it is the free barium atoms that produce the improved electron emissions from these filaments, the presence of anything that will restore Ba to BaO will de-activate the electron emitter. Thus, filaments that are activated by alkaline earth oxides are not stable in the presence of oxygen. A pressure of oxygen as low as 10 mm of mercury is reportedly enough to decrease electron emission by a factor of as compared with an oxygen-free environment. A pressure of lO' mm mercury of oxygen is reportedly sufficient to poison the coating completely. In the plasma furnace, however, with a basically argon atmosphere, the low work function material performed with no apparent loss in effectiveness even though the oxygen impurity was on the order of 0.1 mm Hg. It will be recognized that oxygen at pressures of 0.1 mm Hg is a hundred times greater than that reported to cause de-activation of oxide-coated electron emitters. Accordingly, it is believed that a person skilled in the art of electron tube emission sources would not have predicted what has been observed in the plasma furnace. In fact, such a person would probably have predicted the opposite of what has been observed.

ln addition to de-activation of oxide coating on filaments due to the presence of oxygen, prolonged exposure to CO has been reported to completely de-activate such electron emitters. Furthermore, besides chemical de-activation, deterioration of the coating through evaporation or structural failure (with subsequent separation of the coating from the cathode) due to mechanical shock, etc., will cause the efficiency of the electron emission to fall off rapidly. One example reported at page 838 of the Journal of Applied Physics, in an article entitled, The Properties of Oxide-Coated Cathodes, by John P. Blewett, volume 10, December, 1939, reveals that removal of the activated coating from 50 percent of the metal surface can result in a thousand-fold decrease in electron emission. ln contrast to electron tube experience, the plasma furnace has performed satisfactorily when as little as 10 percent of the cavity walls are coated with the low work function material, and it is possible to even sustain a plasma with gas that is (at least initially) pure CO Since some low work function materials do vaporize, and in their vapor state they can leak out of the furnace, it is possible for an initially adequate quantity of material to eventually become less than adequate to support stability of the plasma, etc. This deficiency can be readily corrected, however, by the simple expedient of opening a port or the like in a cavity wall and dropping in some replacement material. The entire operation can be about as casual as that of a cook dropping a pinch of salt into a pot of bland stew. Alternatively, the low work function material may be prepared in the shape of a solid rod and utilized like a probe which extends into the furnace cavity, and which is periodically pushed a little farther into the cavity at a rate which is commensurate with the rate at which that end of the probe which is exposed to the plasma is gradually consumed through vaporization. Such a configuration is illustrated in FIG. 6.

While the low work function material is beneficial in that it lowers the temperature at which a plasma can be stabilized, it is also particularly beneficial in lowering the minimum re-start temperature, i.e., the temperature at which a plasma can be re-established in a hot furnace by merely applying voltage between two widely separated electrodes, and without the requirement of bringing the electrodes together and creating a conventional arc therebetween, etc. For example, as stated in the Beasley et al. patent, re-starts could not be accomplished below about 2800F. However, with the same furnace and gas, but with the low work function material in the cavity, re-starts have been obtained as low as approximately 1650F. This temperature is about 350F above the minimum temperature (roughly 1300F) at which a stable plasma can be maintained indefinitely after the furnace temperature has been lowered from an operating temperature of at least l650F. This differential of about 350F with argon is understandable when one considers that reestablishing a plasma is more difficult than merely sustaining a plasma, just as it takes more energy to initiate an arc discharge across an air gap than it does to sustain the discharge after it is established. Correspondingly dramatic reductions in critical temperatures have been observed in other gaseous plasmas, both in pure and in mixed gases.

As suggested in an earlier paragraph, the marked decrease in the minimum temperatures that are required for establishing a plasma, permitting re-starts,

and sustaining a plasma after it has once been established, are perhaps best explained on some theory of artificially increasing the supply of free electrons throughout the cavity, i.e., providing electrons from a source other than the cavity wall, the electrodes, and the gaseous atmosphere in the cavity. Accordingly, this theory may be said to be the theory which is adopted to explain the surprising results reported herein; that is, this is the theory which is adopted insofar as there is a compelling necessity to adopt some theory for the purpose of promoting and understanding of the invention, etc. If it develops, however, that another theory is later found to be more plausible as to the manner of operation of the low work function material, it will be understood that it is not intended to limit the appended claims by reference to a theory of operation that is later found to be misleading to any extent.

What is claimed is:

1. The method of promoting the establishment and sustenance of a gaseous plasma between spaced electrodes in a heated cavity comprising the steps of:

placing in a gas-filled cavity a quantity of material having a work function no greater than 3, said quantity of material being at least as much as would be required to cover the walls of the cavity with about one milligram per square centimeter of cavity wall;

heating the gas between the electrodes to at least l300F; and discharging electrical energy across the gap between the spaced electrodes.

2. The method of obtaining restarts in gas-filled arc plasma furnaces after a plasma condition has been interrupted, comprising the steps of:

establishing and maintaining the temperature in the furnace cavity at approximately 1650F or higher; providing and maintaining a quantity of material having a work function no greater than 3 in the cavity of the furnace, said quantity being at least as much as that quantity of material which would be required to cover the walls of the cavity with about one milligram of material per square centimeter of cavity wall; and

establishing a suitable electrical potential across the gap between two spaced electrodes in the furnace cavity.

3. The method of sustaining a gaseous plasma in a cavity at temperatures as low as l300F, consisting of the steps of:

initially establishing a gaseous plasma between spaced electrodes in the cavity;

providing and maintaining a quantity of low work function material in the cavity, said quantity being at least as much as that quantity of material which would be required to cover the walls of the cavity with about one milligram of material per square centimeter of cavity wall;

establishing and maintaining the temperature in the cavity at least as high as about l300F; and establishing and maintaining a suitable electrical potential across the spaced electrodes.

4. The method as claimed in claim 3 wherein the low work function material is maintained in the cavity by the steps of:

preparing the low work function material in the form of a substantially solid rod; and

pushing the rod into the cavity at a rate which is equal to the rate at which the rod is eroded during use.

5. The method as claimed in claim 3 wherein a sufficient quantity of low work function material is maintained in the cavity by continually resupplying the cavity with fresh, particulate material at a rate which is at least equal to that rate at which said material is lost from the cavity.

6. The method of maintaining a substantially constant temperature in a plasma furnace having certain inherent heat losses when a gaseous plasma condition exists in the furnace, consisting of the steps of:

establishing a desired nominal temperature in the furnace by initially adjusting the flow of current between electrodes and through the plasma, whereby a certain current flow is established; and

thereafter constantly maintaining the flow of current through the plasma at said certain level associated with the desired nominal temperature, such that the heating power given by the relationship P=l R is varied only as a result of changes in the resistance of the plasma are, where l is said certain current and R is the resistance of the plasma arc, whereby the temperature in the furnace inherently tends to return to the nominal temperature in the event that temporary deviations from said nominal temperature occur.

7. The method of establishing a gaseous plasma state in any one or mixtures of certain gases, comprising the steps of:

establishing an electric arc between two spaced electrodes in a cavity filled with any one or a mixture of gases including the noble gases, air, oxygen, carbon dioxide, nitrogen, ammonia, methane, and acetylene; and 

2. The method of obtaining restarts in gas-filled arc plasma furnaces after a plasma condition has been interrupted, comprising the steps of: establishing and maintaining the temperature in the furnace cavity at approximately 1650*F or higher; providing and maintaining a quantity of material having a work function no greater than 3 in the cavity of the furnace, said quantity beinG at least as much as that quantity of material which would be required to cover the walls of the cavity with about one milligram of material per square centimeter of cavity wall; and establishing a suitable electrical potential across the gap between two spaced electrodes in the furnace cavity.
 3. The method of sustaining a gaseous plasma in a cavity at temperatures as low as 1300*F, consisting of the steps of: initially establishing a gaseous plasma between spaced electrodes in the cavity; providing and maintaining a quantity of low work function material in the cavity, said quantity being at least as much as that quantity of material which would be required to cover the walls of the cavity with about one milligram of material per square centimeter of cavity wall; establishing and maintaining the temperature in the cavity at least as high as about 1300*F; and establishing and maintaining a suitable electrical potential across the spaced electrodes.
 4. The method as claimed in claim 3 wherein the low work function material is maintained in the cavity by the steps of: preparing the low work function material in the form of a substantially solid rod; and pushing the rod into the cavity at a rate which is equal to the rate at which the rod is eroded during use.
 5. The method as claimed in claim 3 wherein a sufficient quantity of low work function material is maintained in the cavity by continually resupplying the cavity with fresh, particulate material at a rate which is at least equal to that rate at which said material is lost from the cavity.
 6. The method of maintaining a substantially constant temperature in a plasma furnace having certain inherent heat losses when a gaseous plasma condition exists in the furnace, consisting of the steps of: establishing a desired nominal temperature in the furnace by initially adjusting the flow of current between electrodes and through the plasma, whereby a certain current flow is established; and thereafter constantly maintaining the flow of current through the plasma at said certain level associated with the desired nominal temperature, such that the heating power given by the relationship P I2R is varied only as a result of changes in the resistance of the plasma arc, where I is said certain current and R is the resistance of the plasma arc, whereby the temperature in the furnace inherently tends to return to the nominal temperature in the event that temporary deviations from said nominal temperature occur.
 7. The method of establishing a gaseous plasma state in any one or mixtures of certain gases, comprising the steps of: establishing an electric arc between two spaced electrodes in a cavity filled with any one or a mixture of gases including the noble gases, air, oxygen, carbon dioxide, nitrogen, ammonia, methane, and acetylene; and heating the gas between the electrodes to a temperature of approximately 1650*F in the presence of a quantity of low work function material, said material having a work function no greater than
 3. 